Fuel cell systems capable of high-efficiency, small-scaled power generation have been heretofore developed as distributed power generation equipment having high energy utilization efficiency. A primary reason for this is that a system for utilizing heat energy generated when the fuel cell systems generate electric power is easy to construct.
A fuel cell system has a fuel cell as the main body of its power generating section. The fuel cell is an electric cell that directly converts the chemical energy of a fuel (hydrogen) and an oxidant (oxygen) into electric energy through a specified electrochemical reaction. Therefore, in a fuel cell system, the fuel cell is supplied with hydrogen serving as a fuel and oxygen serving as an oxidant during power generation. In the fuel cell, the specified electrochemical reaction that utilizes the supplied hydrogen and oxygen proceeds so that the chemical energy of the hydrogen and oxygen is directly converted into electric energy by this specified electrochemical reaction, while heat and water are generated. Then, the electric energy generated through the specified electrochemical reaction in the fuel cell is supplied from the fuel cell system to the load. The heat, which is generated as the electrochemical reaction proceeds, is recovered by cooling water circulating within the fuel cell to keep the temperature of the fuel cell within a specified range. The heat, which has been recovered, is used in a heat exchanger etc. possessed by the fuel cell system to supply hot water etc. A typical fuel cell system has a hydrogen generator that generates a hydrogen-rich reformed gas. This reformed gas is fed to the fuel cell as a substantial fuel. A typical fuel cell system also has an air feeder by which air is supplied to the fuel cell as a substantial oxidant.
A fuel cell system has (i) a cooling water tank for storing cooling water used for cooling the fuel cell that generates heat during power generation, in order to keep the temperature of the fuel cell within a specified range; (ii) a pump for feeding the cooling water stored in the cooling water tank to a flow path within the fuel cell; (iii) a water purifier for purifying the cooling water fed by the pump before it is supplied to the flow path within the fuel cell; and (iv) a heat exchanger that is provided for the purpose of utilizing the heat of the cooling water for hot water supply etc., which cooling water has risen in temperature and has been discharged from the fuel cell. Herein, the water purifier includes an ion exchange resin (or ion-removal filter) in order to remove conductive ions such as metal ion that are likely to dissolve in the cooling water within the heat exchanger and the fuel cell. While the fuel cell system is in power generation operation, the ion exchange resin is supplied with the cooling water that has passed through the fuel cell, the heat exchanger and others. The conductive ions such as metal ion which have dissolved in the cooling water are removed by the ion exchange resin, and the cooling water from which the conductive ions have removed are supplied to the fuel cell, so that short circulation within the fuel cell caused by the conductive ions can be prevented.
However, there is a problem that most of the ion exchange resins that can be used for the water purifier are comparatively low in heat resistance. This problem is significant particularly where a minus ion exchange resin is used as the ion exchange resin. On the other hand, the temperature of the fuel cell rises to about 70° C. to 80° C. during power generation even when a proton-exchange membrane fuel cell is used as the fuel cell. Therefore, the temperature of the cooling water, which is circulated by the pump to cool the fuel cell, also rises to about 70° C. to 80° C., if the heat recovered by the cooling water in the heat exchanger etc. is not satisfactorily used. If the temperature of the cooling water supplied to the water purifier by the pump exceeds the allowable temperature limit of the ion exchange resin, the degradation of the ion exchange resin due to heat escalates, shortening the service life of the ion exchange resin.
As an attempt to solve the above problem, there has been proposed a fuel cell system capable of effectively removing the conductive ions dissolved in the cooling water without adversely affecting the service life of the ion exchange resin (e.g., Patent Document 1).
For effective removal of the conductive ions dissolved in the cooling water without adversely affecting the service life of the ion exchange resin, this proposed fuel cell system has, similarly to the conventional fuel cell system, (i) a cooling water tank for storing cooling water used for cooling the fuel cell; (ii) a water purifier for purifying the cooling water fed by the pump before it is supplied to the flow path within the fuel cell; and (iii) a heat exchanger that is provided for the purpose of utilizing the heat of the cooling water for hot water supply, which cooling water has been supplied by the pump and discharged from the fuel cell. The proposed fuel cell system further includes: (i) a condensed water tank for storing water discharged from the fuel cell during power generation; (ii) a second pump and water feed passage which are for supplying the water stored in the condensed water tank to the cooling water tank; (iii) a water purifier for purifying the water supplied from the second pump before it is supplied to the cooling water tank; and (iv) a water exhaust passage for discharging excessive cooling water in the cooling water tank to the condensed water tank.
In the proposed fuel cell system, when starting and/or terminating the power generating operation of the fuel cell system, the water stored in the condensed water tank is pumped up into the cooling water tank by activating the second pump after purified by the water purifier. Further, the second pump is activated during the power generating operation to thereby circulate water between the condensed water tank and the cooling water tank through the water feed passage, the water purifier and the water exhaust passage.
According to the proposed fuel cell system, even if the cooling water collected from the cooling water tank into the condensed water tank has a high temperature of 70° C. or more, the temperature of the water discharged from the fuel cell and collected into the condensed water tank is about 40° C. and therefore the temperature of the water supplied from the condensed water tank to the water purifier can be prevented from exceeding the allowable temperature limit of the ion exchange resin. With such an arrangement, the conductive ions dissolved in the cooling water can be removed without adversely affecting the service life of the ion exchange resin. In addition, by circulating water between the condensed water tank and the cooling water tank through the water purifier, continuous water purification can be carried out so that the quality of the cooling water can be maintained during the power generating operation of the fuel cell system.
To sum up, the above proposal enables provision of a fuel cell system capable of removing the conductive ions dissolved in the cooling water without adversely affecting the service life of the ion exchange resin.    Patent Document 1: Japanese Kokai Patent Publication No. 2002-141095